Update docs: how to handle process cancellation

[DanielTuran]

In 2.0.0-rc.1.25451.107 CancellationTokens are not working by default.

RootCommand rootCommand = new();
rootCommand.SetAction(async (parseResult, cancellationToken) => {
	cancellationToken.Register(() => Console.WriteLine("Cancellation requested"));
	await Task.Delay(10000, cancellationToken);
	return 0;
});
await rootCommand.Parse(args).InvokeAsync();

Please write docs on how properly handle process cancellation to gracefully exit the app.

Here are some scenarios where we might want to gracefully exit app:

• user invoked CLI -> user pressed Ctrl+C
• kubernetes job -> job cancelling by k8s controller
• windows service -> service stopping

[KalleOlaviNiemitalo]

It's issue #2671, already fixed in #2672.

Re "windows service -> service stopping": that one can be handled via Microsoft.Extensions.Hosting.WindowsServices.WindowsServiceLifetime.

dotnet/runtime#119257 may also be relevant.

[DanielTuran]

Still I think I should be mentioned in the docs, that setting ProcessTerminationTimeout has an effect on cancellation token being thrown. It's not obvious at all.